Display Screen stays black, but computer still works good

My iBook G3 12" 600Mhz run always top but now he has a problem!

When I start the machine he run normal, only the Display screen stay Bback and I can see nothing more. (Running OS X 10.4.11)

The Problem begins with normal Start, Display works good but 10 Min. Later Display screen change alone black under working? This was 2-3 times and now when I start the iBook the Display screen shows nothing more only thework's good Hard Drive, Light's on Keyboard.....etc.

I have no idea how to fix this problem maybe somebody know the answer.

I don't want change the display if the problem is on a other place (example graphics chip)

Hopeful for Help Thank's a lot

Konrad/ Germany